Direct measurement of surface interactions experienced by sticky microcapsules made from environmentally benign materials

We present a study combining experimental measurements, theoretical analysis, and simulations to investigate core-shell microcapsules interacting with a solid boundary, with a particular focus on understanding the short-range potential energy well arising from the tethered force. The microcapsules, fabricated using a Pickering emulsion template with a cinnamon oil core and calcium alginate shell, were characterized for size (∼5-6μm in diameter) and surface charge (∼-20mV). We employed total internal reflection microscopy and particle tracking to measure the microcapsule-boundary interactions and diffusion, from which potential energy and diffusivity profiles were derived. The potential energy profile characterizing the normal interaction was analyzed and simulated by considering electrostatic, gravitational, van der Waals, and tethered forces, while the lateral diffusivity was compared to that of a solid particle-boundary interaction, inclusive of hydrodynamic forces. The diffusivity was represented as a normalized diffusion coefficient to eliminate the impact of fluid viscosity. The normalized diffusion coefficient of polymer-shell microcapsules (∼0.02) was found to be an order of magnitude smaller than that of solid polystyrene particles (∼0.2). The microcapsule sampled a potential well consisting of two distinct minima, as observed experimentally and supported by analytical expressions and Brownian dynamics simulations. A critical tethered height h_ct=46.3nm and the alginate radius of r_g=32.1nm were obtained from fitting our model to experimental data. This work concludes that these benign core-shell microcapsules interact with a nearby boundary via a transient tethering interaction, overall producing a mild 'sticky' interaction that would likely be beneficial for applications in consumer products.